111

777

+ 999

can you strike out 6 of these digits so that the total of the remaing numbers shall be 20?

Since 111, 777, and 999 are all bigger than 20, make them smaller. Take out one 1, two 7, and two 9 (5 numbers) : 11 + 7 + 9 ≠ 20 (it equals 27). Now take out number 7. (If you take out number 1 [1+7+9=17, not 20] and of you take out number 9 [11+7=18, not 20].

11 + 9 = 20 (you took out numbers: 1, 7, 7, 7, 9 and 9 [6 numbers]).